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Modelica System Modelling & Pilot Plant Design for Thermal Energy Storage


The objective of this project is to assist the team at the National Renewable Energy Laboratory (NREL) to design and model a grid scale energy storage system for sustainable renewable energy integration. This project began in August of 2019 and was motivated by the clear need for better renewable energy infrastructure within the United States. Previous groups who have worked on this project have mainly focused on conceptual design and lab scale testing and have created a strong foundation for the project.

This Fall 2020 – Spring 2021 Senior Design team, Team TES-ting, built off this progress and worked to develop Modelica simulations of the power island and particle handling components to analyze and size various plant components and to build a Solidworks model of the pilot plant located in Arvada, CO. These simulations and CAD models will help NREL to verify and ultimately implement this energy storage system.

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Team Members

  • Clayton Beagley
  • Daniel Celvi
  • Patton George
  • Madison Heeg
  • Drake Minter
  • Tyler Pritchard
  • Ian Pudney

Project and Technical Advisors

  • Adam Duran, Project Advisor
  • Dr. Xingchao Wang, NREL
  • Patrick Davenport, NREL
  • Jeffrey Gifford, NREL

The Client

  • Dr. Zhiwen Ma, NREL
  • Dr. Ruichong Zhang, CSM


A tremendous thank you to the entire NREL team for providing us expertise every step of the way!

Thank you Instr. Adam Duran for your insightful guidance these past eight months.

If the embedded video does not work, please click the here.

Design Approach

Modelica is an open-source modeling software in which users can use a graphical user interface as well as backend code to model a variety of components, either from existing libraries or custom-made components.

After an initial literature review, the first attempts at Modelica were primarily focused on creating functional components that were similar in operation to components that are a part of the larger thermal energy storage system. This process began by creating black box diagrams to understand the key features of each component. After such understanding, modelling in Modelica began. Due to the high learning curve of Modelica, this took considerable time and iterations.

When some components were functioning, the focus turned to attempting to integrate multiple components, and making them more robust. This often involved ensuring the use of physically realistic starting and boundary conditions, and understanding what values were needed by Modelica to properly model the function.

Once components were more robust and could handle changes in conditions that still fit within physical limits, modifications began. A major modification was the development of a silica medium, as many libraries use water which is not physically possible at this system’s operating conditions. In addition, within the components, code was occasionally altered to achieve the required functionality of the machine.

Finally, when components were operating properly and robust, input and output values from the Excel ENDURING tool were used to validate results, along with software such as Mathcad and MATLAB, to ensure components were being modeled in ways that made physical sense.


Work on the pilot plant began with a visit to the proposed site to gain an understanding of the scale, to begin visualization of where components might fit, and to see what existing structures could be repurposed for this project.

Following the site visit, the team began an analysis on the engineering drawings of the existing structure to determine what information was provided in the drawings and where reasonable assumptions could be made. With this information and assumptions, a SolidWorks model of the main frame of the structure was be developed.

The engineering drawings also provided information on the geometry of the existing silo in the structure. The NREL team supplied thermal performance data for the insulation and desired insulation thicknesses. The insulation thickness aided in determining the available capacity for silica and whether one or two silos would be needed to supply enough thermal energy for the proposed 1 MW design.

Design Solution

The team successfully created Modelica models for the particle heater, particle transport, the heat exchanger, and the Brayton cycle power island. All components were validated against hand calculations and the values provided in the ENDURING tool. The block diagram for the Brayton cycle in which the heat exchanger models heat exchange between air and silica is provided for reference.

The team was not able to connect all the components together in a singular overarching model; however, theoretical analysis was performed as if all components were integrated and connected. This was done by modifying the inputs of respective components to reflect the outputs of the component it is supposed to be connected to. Once the outputs became steady, the round-trip efficiency was calculated for the entire storage plant. The round-trip efficiency was found to be 79% which is within acceptable range for energy storage devices.

The final model of the pilot plant shows the primary vertical columns and horizontal beams to provide a tool to visualize and test where components might fit. It does not show minor details such as staircases or provide detail on how beams are connected as those details do not support the primary purpose of the model.

Additionally, a heat transfer analysis on the silo was performed to learn if the proposed insulation thicknesses are sufficiently thick to limit the heat loss to less than 5%.

Next Steps

Given current project progress, next steps involve:

  • Finalizing Modelica component integration
  • Conducting a techno-economic analysis, and
  • Running optimization analyses with the complete Modelica model.


Through current efforts, the team has built a solid foundation of analytical tools upon which these future efforts can be pursued. Further, by developing a library of transition documents and resource videos, the team hopes to efficiently familiarize future technical teams with the current analytical tools of this project so that long learning periods can be avoided.

Once future technical teams achieve familiarity with the OpenModelica software, it is recommended that they focus on conducting sensitivity analyses of key operation parameters within the plant. Key analyses of interest might be increasing the effectiveness of the heat exchanger, lowering the outlet temperature from the gas turbine, lowering the compression ratio of the gas compressor, and reducing the storage temperature of particles in the silo. Such analyses will lend greater insight regarding potential design parameters for the commercial plant slated for development later in the project.

In the end, this project will culminate in the construction of a pilot plant in Arvada, CO, and the development of a TES design for commercial-scale energy storage in North Carolina.

Meet the Team

Clayton Beagley

Born and raised in Bakersfield, California, I planned to attend the Colorado School of Mines straight from high school, but life took me on a different path. I joined the Marine Corps for five years where I had the opportunity to travel the world as an Arabic linguist. Following my time with the Marines, I came to Mines to study mechanical engineering. After graduation, I will begin working as a project engineer at a boron mine in the California desert. When I am not studying, I enjoy golf, soccer, and hiking.

Daniel Celvi

Originally from Illinois, I wandered around a lot once I left 12 years ago, then somehow ended up on a ship. Wouldn’t recommend it if I’m being honest. Moved to Colorado after that, and came to Mines because I could study materials science/engineering, and have been working at NREL for almost 3 years. Outside of school and work, I like to climb, ski, and weight lift.

Patton George

I was born on the shores of Lake Erie in Port Clinton, Ohio and spent the majority of my pre-college years living in Columbus. The Colorado School of Mines was the first college I visited and it set a bar no other university could reach. I will be graduating with a degree in mechanical engineering and a minor in economics. On campus, I have been heavily involved with my fraternity Phi Gamma Delta and the school’s Interfraternity Council. After graduation, I will continue my education at Mines pursuing a non-Thesis Master’s Degree in Nuclear Science and Engineering

Madison Heeg

Houstonian born (basically) and raised, I came to the Colorado School of Mines to escape Houston’s seasons of humid, hot, and hotter. When not studying and living out of the old side of Brown as an electrical engineer with minors in economics and computer engineering, I can often be found soaking up the sun outside CoorsTek or hiking around the area. On campus, I am involved in Blue Key, Tau Beta Pi, Pi Beta Phi, and planning the production and stage of E-days weekend. After graduation, I will be starting my career as a technical sales engineer at Texas Instruments.

Drake Minter

Born in Florida and raised in Kansas, I came to Mines to be near the mountains and to study mechanical engineering with a minor in energy. On campus I’m involved in Solar Decathlon, Mines Energy Club, and Phi Gamma Delta. In my spare time I enjoy rock climbing, disc golfing, and going to the movies. After graduation, I look forward to starting my career as an MEP engineer.

Tyler Pritchard

Originally from Seattle, I came to the Colorado School of Mines to learn how I could make a positive impact on the world as a mechanical engineer focusing on energy applications. Outside of the classroom, I enjoy biking, cooking, hiking, and camping; if it’s outside, I’ll probably enjoy it! After Graduation, I look forward to starting a M.S. in Energy Engineering at IMT-Atlantique in France as part of the Erasmus Mundus Joint Master’s Degree program.

Ian Pudney

Born and raised outside of Tacoma, Washington, I came to Mines in order to experience the incredible outdoor activities that Colorado offers. I decided to pursue electrical engineering after growing up and hearing about the cool projects that my dad would work on as an electrician in Seattle. Outside of school I enjoy snowboarding, and camping, and although I have enjoyed Colorado, I am excited to be heading back to Washington after graduation in order to begin my career as an electrical engineer.